IB Chemistry Bonding & Structure centres on a manageable set of testable ideas: bonding models, Lewis structures, molecular geometry, polarity, intermolecular forces, material properties, and structure-property explanations. To score consistently, you must do more than recognize a bonding type. You need to connect particles and forces to observable properties using precise chemical language.
In the current course, first assessed in 2025, this material appears under Structure 2: Models of bonding and structure. It includes the ionic, covalent and metallic models, followed by the application of those models to materials. The official IB Chemistry course page explains that structure and reactivity are the two organizing concepts of the course.
What IB Chemistry tests in Bonding & Structure
The examinable content is organized into four areas:
| Syllabus area | Central questions |
|---|---|
| S2.1 The ionic model | How do ions form, arrange themselves and produce characteristic properties? |
| S2.2 The covalent model | How do electron pairs determine bonds, shape, polarity and intermolecular attractions? |
| S2.3 The metallic model | How does a lattice of cations and delocalized electrons explain metallic properties? |
| S2.4 From models to materials | How can bonding models explain alloys, polymers and materials with mixed bonding character? |
The IB recommends more teaching time for this topic at HL because HL students study additional covalent bonding concepts, including resonance, formal charge, sigma and pi bonds, and hybridization. The official Chemistry curriculum update also confirms that questions may appear across Paper 1A, Paper 1B and Paper 2 rather than being isolated in a single bonding section.
The three principal bonding models
| Model | Particles and attraction | Typical structure | Key properties |
|---|---|---|---|
| Ionic | Electrostatic attraction between oppositely charged ions | Three-dimensional ionic lattice | High melting point; brittle; conducts when molten or aqueous |
| Covalent | Attraction between a shared electron pair and both positive nuclei | Molecules or covalent networks | Properties depend strongly on structure and intermolecular forces |
| Metallic | Attraction between a lattice of cations and delocalized electrons | Giant metallic lattice | Conductive, malleable and generally strong |
These are models, not perfectly separate categories. Bonding forms a continuum, so substances can have different proportions of ionic, covalent and metallic character. This is the basis of the bonding triangle studied in S2.4.
Ionic bonding and lattice properties
Metal atoms commonly form cations by losing electrons, while non-metal atoms form anions by gaining electrons. The ionic bond is the electrostatic attraction between oppositely charged ions throughout a lattice, not a single directional bond between two isolated particles.
A strong exam explanation follows a particle-to-property chain:
- Ionic compounds have high melting points because many strong electrostatic attractions must be overcome.
- Solid ionic compounds do not conduct because their ions occupy fixed lattice positions.
- Molten or aqueous ionic compounds conduct because charged ions can move.
- Ionic crystals are brittle because displacement can bring like-charged ions together, causing repulsion and fracture.
Do not claim that every ionic compound is soluble in water. Solubility depends on the balance between attractions within the lattice and attractions formed between ions and solvent molecules.
Covalent bonding and Lewis structures
A covalent bond is the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. A single, double or triple bond contains one, two or three shared electron pairs respectively. As bond multiplicity generally increases, bonds become shorter and stronger.
For a Lewis structure:
- Count all valence electrons, adjusting for charge.
- Select a plausible central atom, usually the least electronegative except hydrogen.
- Add single bonds.
- complete outer-atom octets.
- Place remaining electrons on the central atom.
- Introduce multiple bonds if the central atom lacks an octet.
- Enclose an ion in brackets and show its charge.
A coordination bond is covalent, but both electrons in the shared pair originate from the same atom. Once formed, it behaves like an ordinary covalent bond.
VSEPR shapes and molecular polarity
The valence shell electron pair repulsion model, or VSEPR, predicts shape from repulsion between electron domains around a central atom. A single, double or triple bond counts as one electron domain. Lone pairs repel more strongly than bonding pairs, so they usually compress bond angles.
| Electron domains | Bonding domains | Lone pairs | Molecular geometry | Ideal angle |
|---|---|---|---|---|
| 2 | 2 | 0 | Linear | 180° |
| 3 | 3 | 0 | Trigonal planar | 120° |
| 3 | 2 | 1 | Bent | Less than 120° |
| 4 | 4 | 0 | Tetrahedral | 109.5° |
| 4 | 3 | 1 | Trigonal pyramidal | About 107° |
| 4 | 2 | 2 | Bent | About 104.5° |
Bond polarity results from a difference in electronegativity. Molecular polarity depends on both bond polarity and geometry. Carbon dioxide has polar C=O bonds but is non-polar overall because its equal bond dipoles cancel in a linear, symmetrical molecule; water is polar because its bent geometry prevents cancellation.
For focused review, use the Structure 2 notes and learning resources and practise drawing the shape before deciding whether the dipoles cancel.
Intermolecular forces and physical properties
IB questions frequently ask students to compare boiling points, volatility or solubility. The relevant attraction must be named and connected to the energy required to separate particles.
- London dispersion forces act between all atoms and molecules. Their strength generally increases with electron-cloud size and polarizability, and can also depend on molecular surface contact.
- Dipole-dipole attractions occur between permanent dipoles in polar molecules.
- Dipole-induced dipole attractions arise when a permanent dipole distorts a neighbouring non-polar electron cloud.
- Hydrogen bonding occurs when hydrogen bonded to N, O or F is attracted to a lone pair on N, O or F in another particle.
For substances of comparable molar mass, the syllabus gives the general order London dispersion < dipole-dipole < hydrogen bonding. However, a large, highly polarizable non-polar molecule can have stronger total attractions than a much smaller polar molecule, so avoid applying the order without checking molecular size.
An effective boiling-point answer states the strongest relevant attraction, compares its strength or extent, and explains that more energy is required to separate molecules. Never write that covalent bonds are broken during boiling because the molecules remain intact.
Covalent networks and metallic structures
Covalent network substances contain atoms joined by covalent bonds throughout an extended structure. Diamond is hard and has a high sublimation temperature because strong covalent bonds extend in three dimensions. Graphite has strong bonds within layers but weaker attractions between layers, allowing them to slide; delocalized electrons also make graphite electrically conductive.
Metallic bonding involves attraction between positive metal ions and delocalized electrons. Metals conduct because these electrons can move through the lattice. They are malleable because layers of ions can shift while the non-directional electrostatic attraction is maintained.
Metallic bonding generally strengthens with greater ionic charge and smaller ionic radius because charge density increases. HL students should also recognize the contribution of delocalized d-electrons in transition metals. A worked lesson on metallic bonding and metallic properties can help make this explanation more concrete.
Higher Level bonding ideas
HL questions add several connected models:
- Resonance occurs when more than one valid Lewis structure differs only in electron placement. The actual structure is a resonance hybrid with delocalized electrons.
- Formal charge is calculated as valence electrons minus non-bonding electrons minus half the bonding electrons. Preferred structures usually minimize formal charges and place negative charge on more electronegative atoms.
- A sigma bond forms through head-on orbital overlap. A pi bond forms through sideways overlap, so a double bond contains one sigma and one pi bond.
- Hybridization relates electron-domain geometry to orbitals: two domains correspond to sp, three to sp² and four to sp³.
- Some central atoms can have an expanded octet, while species with too few electrons or an odd number of electrons can depart from the octet rule.
Use the bonding and structure flashcards to learn definitions, but follow them with written questions requiring diagrams and explanations.
How examiners phrase bonding questions
Command terms determine the required depth. The current IB Chemistry guide and command-term glossary distinguishes brief recall from developed reasoning.
| Command term | What to provide |
|---|---|
| State | A brief answer without explanation |
| Draw | An accurate diagram, including lone pairs, charges or labels where relevant |
| Deduce | A conclusion based on information provided |
| Distinguish | Explicit differences between both concepts |
| Explain | A causal chain showing why the result occurs |
| Compare | Similarities or differences referring to both substances throughout |
For an explain question about melting point, use this structure: identify the structure, identify the particles and attractions, compare their strength, then connect this to energy. For example, silicon dioxide has a much higher melting point than carbon dioxide because SiO₂ has a covalent network in which many strong covalent bonds must be broken, whereas CO₂ is molecular and only intermolecular attractions are overcome.
Common mark-losing errors include confusing intermolecular forces with covalent bonds, calling ionic substances molecules, omitting lone pairs from Lewis structures, and deciding polarity from bonds without considering geometry.
Turning knowledge into exam marks
Study the content in three passes:
- Learn definitions, structures and standard geometries.
- Practise short questions by subtopic in the Bonding and Structure Questionbank, using Jojo AI feedback to identify missing chemical language.
- Watch IB Chemistry past-paper video walkthroughs to see how a complete method is converted into markscheme points, then attempt a similar question without support.
Worked video solutions are particularly useful for multi-stage tasks involving Lewis structures, VSEPR, polarity and intermolecular forces. Pause before each step, predict what should be written, and compare the wording with the demonstrated solution rather than watching passively.
Conclusion
IB Chemistry Bonding & Structure is most manageable when every property is traced back to structure, particles, attractions and energy. Secure the ionic, covalent and metallic models first, then connect Lewis structures to geometry, polarity and intermolecular forces; HL students should add resonance, formal charge, sigma and pi bonding, and hybridization.
RevisionDojo's Structure 2 notes and Flashcards can support recall, while the Questionbank, Jojo AI feedback and per-question past-paper video solutions are better suited to developing exam-ready explanations. Use those tools to practise the full reasoning chain rather than memorizing isolated property lists.
Sources and referenced URLs
- IB Chemistry in the Diploma Programme
- Official IB Chemistry curriculum update
- IB Chemistry guide for first assessment 2025
- IUPAC definition of an ionic bond
- IUPAC definition of a covalent bond
- RevisionDojo IB Chemistry resources and past-paper videos
- RevisionDojo Structure 2: Models of bonding and structure
- RevisionDojo Bonding and Structure notes
- RevisionDojo Bonding and Structure Questionbank
- RevisionDojo Structure 2 Flashcards
- RevisionDojo metallic bonding video lessons
